BEIRUT/WASHINGTON: U.S. Defense Secretary Jim Mattis said Friday that while he does not have evidence of the chemical nerve agent sarin being used by the Syrian government, the United States was looking into reports about its use and was concerned. Mattis told reporters the Syrian government had repeatedly used chlorine as a weapon.

“We are even more concerned about the possibility of sarin use ... I don’t have the evidence, what I am saying is, that other groups on the ground, NGOs, fighters on the ground, have said that sarin has been used, so we are looking for evidence,” Mattis said.

A deadly sarin attack on a rebel-held area in April prompted President Donald Trump to order a missile strike last year on the Shayrat air base, from which the Syrian operation is said to have been launched.

“We are on the record and you all have seen how we reacted to that, so they would be ill-advised to go back to violating the chemical [weapons] convention,” Mattis said.

The Syrian government has said it has not carried out any of the attacks.

U.S. officials have said the Syrian government may be developing new types of chemical weapons, and President Donald Trump is prepared to consider further military action if necessary.

Last April, the U.S. launched several dozen Tomahawk cruise missiles at the Shayrat air base in response to what it called illegal Syrian use of chemical weapons.

In his remarks, Mattis alluded to the April attack, saying, “so they’d be ill-advised to go back to violating” the international prohibition on the use of chemical weapons.

President Bashar Assad is believed to have secretly kept part of Syria’s chemical weapons stockpile despite a U.S.-Russian deal under which Damascus was supposed to have handed over all such weapons for destruction in 2014, the officials said.

Meanwhile, bombardment by the Syrian regime killed at least 11 civilians Friday in the rebel-held enclave of Eastern Ghouta, east of Damascus, the Syrian Observatory for Human Rights reported.

A barrage of rockets fired at two Turkish border villages from a Kurdish enclave in Syria killed two people and wounded 19 others, Turkish officials said.

Turkey launched a cross-border offensive into Afrin on Jan. 20 to rout the Syrian Kurdish militia group it says is linked to insurgents fighting inside Turkey.

The Observatory said that since the assault began, Turkish troops and their allies have captured 15 villages, and 68 civilians, including 21 children, have been killed.

To the east, Syrian troops and their allies captured the village of Tal Alloush, bringing them within 13 kilometers to the rebel stronghold of Saraqeb, according to the observatory and the government-controlled Syrian Central Military Media.